Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility is monitored such that if the commercial electrical power from the utility fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical power generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator. As is known, most residential electric equipment in the United States is designed to be used in connection with electrical power having a fixed frequency, namely, sixty (60) hertz (Hz).
Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. The frequency of the output power of most prior electrical generators depends on a fixed, operating speed of the engine. Typically, the predetermined operating speed of an engine for a two-pole, stand-by electrical generator is approximately 3600 revolutions per minute to produce the rated frequency and power for which the unit is designed. However, in situations when the applied load is the less than the rated kilowatt load for which the unit is designed, the fuel-efficiency of the engine will be less than optimum. As such, it can be appreciated that it is highly desirable to vary the operating speed of the engine of an electrical generator to maximize fuel efficiency, and thus reduce CO2 emissions, of the engine for a given load. Further, operation of the engine-driven, electrical generator at its predetermined operating speed can produce unwanted noise. It can be appreciated that reducing the operating speed of the engine of an electrical generator to correspond to a given load will reduce the noise associated with operation of the engine-driven, electrical generator.
If the size of the load is substantial, it may be desirable to utilize multiple generators in parallel rather than a single generator to provide power to the load. If multiple generators are utilized and one of the generators fails, the remaining generators are still able to provide a portion of the power required by the load. Further, if the load requirements were to increase, another generator may be added in parallel to supplement the existing capacity of the generators. Replacing a single generator, which already is larger in size than paralleled generators, with an even larger generator would be a significant expense.
If two alternating current (AC) power sources, such as multiple generators, are to be connected in parallel, the AC output voltages must be synchronized otherwise the instantaneous difference in voltage potential may result in current transferred between the two voltage sources. If one of the generators is a single-phase generator, the pulsating torque produced by a single-phase generator may result in some fluctuation in frequency of the power output by the generator. Variations in the load applied to the generator may also cause fluctuation in the frequency of the power output by the generator. Historically, it has been known to mechanically couple the output shafts of the generators to ensure that the generators synchronously supply voltage to the load. However, mechanical coupling adds increased expense and complexity to connecting multiple generators in parallel.
Therefore, it is a primary object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator such that it may be connected in parallel with at least one other generator.
In accordance with one embodiment of the present invention, a method of controlling an engine-driven, electrical generator system configured to be connected in parallel with a second power source is disclosed. The generator system generates a first AC voltage at a first frequency with the engine running at an operating speed, and the second power source provides a second AC voltage at a second frequency. An input signal, corresponding to a measured or estimated value of a phase angle of the second AC voltage, is received at the generator system, and a frequency of the second AC voltage is determined as a function of the input signal. The operating speed of the engine is varied in response to a load thereon. A difference between the frequency of the first AC voltage, generated responsive to the operating speed of the engine, and the frequency of the second AC voltage is calculated and provided as an adjustment frequency. The frequency of the first AC voltage is modified by the adjustment frequency independent of the engine speed.
According to another aspect of the invention, the generator system includes an alternator, which, in turn, includes a rotor having windings and a stator having an output. The output of the stator is connectable to the load. The output of the stator may be connected to an input of an inverter, and an output of the inverter may be connected to the windings of the rotor. The inverter supplies power to the rotor windings at the adjustment frequency. The stator may include a main winding and a quadrature winding and the inverter may include a DC link, where the DC link is connected to the quadrature winding.
According to another embodiment of the invention, a system for synchronizing a first AC voltage generated by a first alternator with a second AC voltage generated by a second alternator is disclosed. An output of the first alternator is connected in parallel with an output of the second alternator to provide power to an electrical load. The system includes an engine configured to rotatably drive a rotor of the first alternator at an operating speed. A controller is configured to receive an input corresponding to a magnitude of the electrical load and to regulate the operating speed of the engine as a function of the magnitude of the electrical load. An inverter is configured to generate an AC voltage at an adjustment frequency for a multi-phase winding on the rotor of the first alternator. The adjustment frequency is the difference between a frequency of the second AC voltage and a frequency of the first AC voltage generated responsive to the operating speed of the engine.
According to another aspect of the invention, an input is configured to receive a signal corresponding to a phase angle of the second AC voltage, and a phase angle of the first AC voltage is synchronized to the phase angle of the second AC voltage. The inverter may include the input configured to receive the signal corresponding to the phase angle of the second AC voltage. The inverter also includes a processor configured to generate the adjustment frequency and varies the adjustment frequency to synchronize the phase angle of the first AC voltage to the phase angle of the second AC voltage. Optionally, the controller may be configured to receive the signal corresponding to the phase angle of the second AC voltage and to generate a frequency reference signal to the inverter. The inverter may set the adjustment frequency to the frequency reference signal, and the controller varies the frequency reference signal to synchronize the phase angle of the first AC voltage to the phase angle of the second AC voltage.